Up to the present time, a call covering party such as a secretary, attendant, etc. has been able to process calls for more than one principal station only by means of selection keys dedicated to each principal station, or by dialing a principal station telephone number, sometimes in conjunction with other feature keys. For example, a secretary who provides call coverage for, say, five executives (principals) typically has a direct station selection (DSS) key field either built into the station console or a separate DSS console associated with the station. In either case, the DSS field typically consists of a field of push-button keys (switches) each individually dedicated to a different one of the principals. In addition, the secretary's station may have a separate key appearance for each of the principal's lines. Thus, for example, if a call arrives for principal A which is unanswered by A, the secretary may depress A's line key, answer the call, take a message, and light a message waiting lamp on A's station by depressing the DSS key associated with A. Alternatively, a secretary may routinely answer a principal's calls and alert the principal to answer only the important calls. A DSS field may be wired for intercom use to a principal in these cases.
Certain types of telephone systems typically use selective processing for covered calls in place of the DSS field. A secretary may screen calls for the principals and alert the principals to answer the important calls by placing the calls on HOLD and by dialing the extension numbers of the appropriate principals on an intercom system.
The DSS type of operation, while satisfactory, is expensive to provide in terms of the number of dedicated switches needed throughout a system, and in some systems in terms of the dedicated wiring required per switch. The selective processing arrangements, on the other hand, require substantially more manual operations on the part of the covering party than the DSS systems to perform the same functions.